Every four years inhabitants of Earth are blessed with an extra day. For many, this day is most remarkable for briefly extending the month of February, a month already notable for its truncated length. It's all very low-key and unremarkable, sort of like daylight saving time.
Lost in this anti-climax is the truly remarkable reason that leap days exist: because the planet runs on its own time, not ours. This may seem obvious, but in a day and age in which we traverse vast distances at recently unimaginable speeds and are increasingly connected in a way that makes physical space seem obsolete, a humble reminder of the power of nature and natural causes is not so absurd.
A reminder of human domination is less necessary, however in some ways the degree of this impact is still surprising.
In an epoch increasingly referred to as the Anthropocene due to its defining feature, human impact, it's become clear that through climate change we are altering the planet's natural seasonal cycle. In this sense a big reason leap years are observed—to stay in line with the seasons—is no longer applicable in the same way it's been for thousands of years.
"The main purpose of the leap year it to keep our calendar aligned with Earth's position in its orbit around the sun and with the resulting seasonal cycles," Burkhard Militzer, an associate astronomy professor at UC Berkeley, told me. "Our climate is changing whether we choose to change our calendar or not."
365.242189 Days or Why Leap Days Exist
The basic scientific explanation for why for every three years with 365 days there's one with 366 is pretty simple: it actually takes the planet 365 days and six hours, or to be precise 365.242189 days, to complete a full revolution around the sun. If the calendar wasn't updated to adjust for this discrepancy, eventually it would fall out of whack with the natural seasons of the planet, screwing everything up. Over the course of 100 years, without this change the calendar would shift around 24 days in relation to the solar cycle and natural seasons. So if you were born in early spring and you didn't move, you could celebrate your one-hundredth birthday in the depths of winter.
It does actually get a little more confusing though since the exact length of a solar year is actually 11 minutes and 14 seconds less than 365 days and six hours. To adjust to this galactic nuisance, in 1582 it was determined that leap years would be omitted three times every four hundred years. When all the math is said and done, the final rule is that a year cannot be a leap year unless it is evenly divisible by 400. With all these adjustments, it now takes 3,300 years for the calendar year and solar year to diverge by a day.
While there are proponents of changing the way we account for the time it takes the Earth to revolve around the Sun, Douglas Finkbeiner, a professor of astronomy and physics at Harvard, told me he wouldn't recommend it.
"I think of our calendar as being tied to how high the Sun is in the sky," he said. "Mid-summer is when the Sun is the highest and winter when it is the lowest. By tradition, 'Summer' starts around June 21, when the Sun is highest."
Sara J. Schechner, a lecturer in the history of science at Harvard, reiterated that the significance of the arbitrary way the solar cycle is divided up into weeks and months isn't just about temperature.
"The number of hours of light and darkness at different times of the year are due to the Earth’s position as it revolves around the sun and the latitude," she said. "Many biological features that we associate with the seasons are due to light and not just temperature."
"For instance, Christmas is a holiday associated with the winter solstice and the turning point of passing the shortest day of the year," she said.
Ancient Egyptians were the first people to calibrate the calendar to the seasons by adding another day. Several thousand years later, humans have found a way to shift seasons—only not in a way that aligns with any predetermined calendar. As greenhouse gases accumulate in the atmosphere and the planet warms, a number of dramatic seasonal changes will take place, including the earlier arrival of springtime, a hotter and longer summer, and a shorter and less frigid winter.
Early Onset Spring
While tracking the physical impacts of climate change is often a drawn-out and laborious affair, noting the arrival of springtime can be done by simply observing nature and watching leafs unfold, flowers bloom, or birds migrate. According to the 2007 Intergovernmental Panel on Climate Change report, since the late 1970s spring has been arriving about 2.3 to 5.2 days earlier per decade. Make sure you read that correctly: it's not total, but per decade. One recent study determined that in the United States, spring will arrive 3 weeks earlier by the end of the century.
What this all goes to show is that up until recently, humans were forced to adapt to nature in order to survive. At some point in the last 150 years, we figured out how to make nature adapt to us. At some point in the next 150 years this relationship will have to be ironed out, or else maybe it will just keep getting thrown more off-balance. The only thing that's clear is that thousands of years down the line, the Earth will still revolve around the Sun every 365.242189 days. What's unclear is who will be experiencing the revolution.